A current amplifier circuit shown in FIG. 3 has been used for a detecting circuit and operations of this circuit are explained hereunder. When a bias current flowing into the transistors T.sub.5, T.sub.6 is I, a base potential V.sub.b of the transistor T.sub.7 at the normal temperature is expressed as follows. EQU V.sub.b =2.multidot.(KT/q.multidot.ln I/IS)
Where, K is Boltzmann's constant, T is absolute temperature, q is charge. When T=300.degree. K., KT/q becomes 26.times.10.sup.-3 EQU Vb=2(26.times.10.sup.-3 .multidot.ln I/IS)
It is equal to a sum of the base and emitter voltages of transistors T.sub.7, T.sub.8. Therefore, ##EQU1##
This equation can be transformed as follow. ##EQU2##
Here, if IB=IX/h.sub.FE, EQU I.sup.2 =IX.sup.2 /h.sub.FE
Therefore, following relation can be obtained. EQU IX=I.sqroot.h.sub.FE
Namely, a bias current IX of transistor T.sub.8 becomes equal to .sqroot.h.sub.FE times of the bias current I.
Meanwhile, when an input current is i, current gain becomes almost h.sub.FE times.
In above structure, since bias current IX and current gain of output transistor T.sub.8 much depends on h.sub.FE, such structure has been weak with regard to fluctuation of elements and it has also been difficult to design such structure. It has also been a factor to realize integration of this structure.